The invention relates to a method and device for avoiding voice interruptions in a cellular communication system, for instance, in a GSM system.
With a communication via a digital cellular communication system, for instance, the GSM system or the UMTS system (Universal Mobile Telecommunications System) interruptions in voice transmission occur in some cases. One situation in which interruptions occur is when the transmission mode is changed from the full transmission rate, the so-called full rate, to half the transmission rate, the so-called half rate.
Another situation in which interruptions in the voice transmission occur refers to the so-called hand-over, for instance, between cells of a base station. As is generally known, hand-over refers to the automatic delivery of a current voice connection from one cell to another one. In this case, it is detected by first means that a hand-over has to take place, whereupon the connection is switched via a channel in a current cell to another channel in another cell with second means. This is ideally effected in a manner that the user does not realize it, or at least that the interference with the user is reduced to a minimum (xe2x80x9cThe GSM System for Mobile Communicationxe2x80x9d; by: M. Mouly et al; Cell and Sys, Paris).
A method for avoiding the complete disconnection of a voice connection in case of a hand-over is known from GB 2 281 177. The method described therein relates to the switching of channels during transition from one cell to another one. According to the method it is suggested that a different time slot is available for the connection in each cell. When moving from one cell to another, at first both time slots are used simultaneously. A first time slot of the previous connection is thereby available serving the maintenance of the quality of the connection, whereas the second time slot is made available for the new connection.
Thus, it is avoided that a connection is discontinued abruptly and completely, however, also a high channel occupancy occurs. Particularly in view of the nowadays existing small resources of transmission capacities and the large-scale traffic in the individual cells this is a disadvantage, as the channels are too strongly occupied resulting in bad quality during voice transmission as well as in short-time interruptions in the voice transmission due to loss of individual voice data containing segments.
Another known method is disclosed in U.S. Pat. No. 5,577,047, which describes a method for a so-called hand-over in a TDMA (Time Division Multiple Access) system. The system consists of a so-called remote station and at least two base stations. The station thereby measures a time delay between signals sent from the base stations and received by the station. On the basis of the time delay, a possible time slot for an additional TDMA channel is detected. Via said channel, an additional signal is sent from the additional base station to the station. The additional signal contains the same information as the first signal. Both signals received by the station are combined in order to provide for a combined signal for the further processing. In the described method, a transmission rate is selected for making the required additional channels available, which corresponds to half the transmission rate of the preceding transmission in order to provide double the number of channels. For this purpose, a transmission being coded at full transmission rate is switched to an additional one being coded at half the transmission rate. Even though a loss or the discontinuation of a transmission during the hand-over from one cell to another cell are avoided in said described method, the switching between the transmission rates is equally subject to an interruption in the transmission resulting in the restricted quality of the voice transmission.
Accordingly, it is the object of the present invention to provide a method and a device for avoiding the interruption in a voice transmission during hand-over.
Furthermore, it is an object of the invention to provide a method and a device, wherein a switch-over between codings comprising different transmission modes is effected during a voice connection without interrupting the same.
The present invention relates to a method and device for avoiding interruptions in voice transmission in a cellular communication system. The voice data is divided into segments and the segments are associated with the transmissions bursts. A first number of segments are coded with a first transmission mode, while a following segment is coded with a second transmission mode. The unused parts of the associated transmission bursts are utilized for performing additional functions. A random number bit sequence is generated and added to the unused parts of the transmission bursts.
It has thereby proved to be advantageous that the quality of the voice transmission remains constantly in a good condition and that no such interruptions occur, which restrict the sound quality during a connection.
It is equally advantageous that the capacities available are exploited to an optimum, wherein a plurality of voice connections of simultaneous high quality can exist or be built up.
Furthermore, it is an advantage that the method and the device allow a flexible switch-over between transmission processes at different transmission rates, i.e. methods and devices where the segments are coded with different transmission modes, by also allowing high quality of the voice transmission at the same time.
It has proved to be particularly advantageous, during the transmission of signaling information on a FACCH (Fast Associated Control Channel), to use voice segments for a different purpose. Voice segments are not replaced by FACCH segments which result in the loss of the voice segments, but the FACCH are added to the voice segments, wherein the voice quality is not lost or influenced.
It is also advantageous that the invention can be used during both, a hand-over within one cell and a hand-over between different cells.